In the operation of nuclear reactors, it is customary to remove fuel assemblies after their energy has been depleted to a predetermined level. Upon removal, this spent nuclear fuel (“SNF”) is still highly radioactive and produces considerable decay heat, requiring that great care be taken in its packaging, transporting, and storing. Specifically, SNF emits extremely dangerous neutrons (i.e., neutron radiation) and gamma photons (i.e., gamma radiation) in addition to generating an amount of heat, if not properly removed, sufficient to cause damage to at least some the materials of the containers in which it is stored and potentially compromising the integrity of the cask.
It is imperative that these neutrons and gamma photons be contained at all times during transportation and storage of the SNF. It also imperative that the residual decay heat emanating from the SNF have a path to escape to avoid the cask reaching unsafe temperatures. Thus, containers used to transport and/or store SNF must not only safely enclose and shield the radioactivity of the SNF, they must also provide an effective way to remove the heat produced by the SNF. Such transfer and/or storage containers are commonly referred to in the art as casks.
Generally speaking, there are two types of casks used for the transportation and/or storage of SNF, ventilated vertical overpacks (“VVOs”) and thermally conductive casks. VVOs typically utilize a sealable canister that is loaded with SNF and positioned within a cavity of the VVO. Such canisters often contain a basket assembly for receiving the SNF. An example of a canister and basket assembly designed for use with a VVO is disclosed in U.S. Pat. No. 5,898,747 (Singh), issued Apr. 27, 1999, the entirety of which is hereby incorporated by reference. The body of a VVO is designed and constructed to provide the necessary gamma and neutron radiation shielding for the SNF loaded canister. In order to cool the SNF within the canister, VVOs are provided with ventilation passageways that allow the cooler ambient air to flow into the cavity of the VVO body, over the outer surface of the canister and out of the cavity as warmed air. As a result, the heat emanated by the SNF within the canister is removed by natural convection forces. One example of a VVO is disclosed in U.S. Pat. No. 6,718,000 (Singh et al.), issued Apr. 6, 2004, the entirety of which is hereby incorporated by reference.
The second type of casks are thermally conductive casks. In comparison to VVOs, thermally conductive casks are non-ventilated. In a typical thermally conductive cask, the SNF is loaded directly into a cavity formed by the cask body. A basket assembly is typically provided within the cavity itself to guide the square fuel assemblies into the proper location and to secure the SNF in place. As with the VVOs, the body of the thermally conductive cask is designed to provide the necessary gamma and neutron radiation shielding for the SNF. In contrast to VVOs, however, which utilize natural convective forces to remove the heat that emanates from the internally stored SNF, thermally conductive casks utilize thermal conduction to cool the SNF. More specifically, the cask body itself is designed to lead the heat away from the SNF via thermal conduction. In a typical thermally conductive cask, the cask body is made of steel or another metal having high thermal conductivity. As a result, the heat emanating from the SNF is conducted outwardly from the cavity and through the cask body until it reaches the outer surface of the cask body. This heat is then removed from the outer surface of the cask body by the convective forces of the ambient air.
In some instances, the use of VVOs is either not preferred and/or unnecessary. This may be due to the heat load of the subject SNF, the existing set-up/design of the storage facility at which the SNF is to be stored and/or the nuclear regulations of the country in which the storage facility is located. However, existing designs of thermally conductive casks suffer from a number of drawbacks, including without limitation: (1) less than optimal heat removal; and (2) vulnerability to the escape of radiation (i.e., shine). Additionally, existing methods of manufacture and designs of thermally conductive casks allow little to no flexibility in altering cask dimensions without a total redesign of the cask and/or retooling of the manufacturing facility.
Metal casks used to store and/or transport spent nuclear fuel must have the ability to dissipate a large quantity of heat, particularly when the fuel has a relatively high burn-up or a relatively low cooling time. Most of the heat from the cask is rejected to the environment by the lateral cylindrical surface of the cask. These and other deficiencies are remedied by the present invention.